This invention relates to catalytic compositions of matter having a specified metal particle size range deposited thereon and their methods of manufacture. In addition, this invention relates to a process for the catalytic conversion of a synthetic feedstock to hydrocarbonaceous materials by contacing said feedstock with a catalytic composite comprising metal particles deposited thereon in a specific size range.
The size of a metal particle on a support may be very important to developing a catalyst which meets the activity, stability and selectivity requirements of a particular catalytic process. For example, it has been noted that synthesis gas conversion may be improved by controlling the particle size of the active metal on a catalytic support. M. Boudart, J. Phys. Chem., v. 88, (11), 1984. A means for controlling the size of metal particles to be impregnated is through the employment of microemulsions. The term "microemulsions" has been used to describe a variety of multi-component systems. In the instant invention a microemulsion is defined as a thermodynamically stable solution of water, hydrocarbon and surfactant which has the property of transmitting light equally in all directions. The microemulsions contemplated by the present invention are systems in which the hydrocarbon forms a solvent containing water in a reverse micelle, i.e., water pools surrounded by surfactant molecules. Each reverse micelle contains one water pool which may be thought of as an aqueous nuclei or water core. The water cores are isolated from one another by the nonaqueous environment.
The prior art discloses that metal salts may be dissolved into the individual water pools of the microemulsion. For example, in U.S. Pat. No. 4,425,261 reverse micelle microemulsions comprising hexane, pentaethylene glycol dodecylether (PEGDE), water, and respectively platinum, palladium, and rhodium salts were prepared. The metal compounds employed were dissolved and encapsulated in the water core of the reverse micelle. In order to obtain an impregnant of uniform metal particles, the micelle solution was first neutralized with an alkali base such as sodium hydroxide and the metal in the water core of the micelle was subsequently reduced with a reducing agent such as hydrogen and/or hydrazine. The metal particles formed in this way were uniform in size and did not deviate more than .+-.10% in diameter. Only after the metal particles were reduced and metal flakes were formed within the water core could the metals be deposited on an oxide support. In other words, the impregnant disclosed by patentee necessarily requires the active impregnate metal to be present in a reduced valence state prior to contact with any oxide support.
The above described liquid suspension is believed to be the closest prior art disclosing reverse micelle containing impregnant solutions. Although patentee's liquid suspension is functional, it is not readily adaptable to commercial catalyst manufacture. First, the preparation of patentee's liquid suspension requires multiple steps leading to high economic inefficiency. For example, the reuse of starting materials and reagents, a common manufacturing technique, may be inhibited. Also, the impregnation of reduced metal particles on an oxide support does not generally result in an acceptable catalyst because the requisite thermal stability cannot be achieved. The transfer of colloidal metallic particles onto a support surface lacks sufficient metal-support chemical interaction, a feature which is generally accepted as a requisite for good catalytic action. Desirable metal-support interaction is gained usually when the metallic state is achieved in the presence of the support during catalyst finishing. In addition, the colloidal metallic particles in the patentee's description may also agglomerate during impregnation or transfer onto the surface of a support, leading to broad metallic size distribution.
The catalyst composition contemplated in the present invention is prepared from an impregnant medium which avoids these problems by maintaining the metal impregnate ions contained in the water core of the reverse micelle in a nonreduced state. In accordance with the process for catalyst manufacture employing the reverse micelle containing liquid impregnant medium of the present invention and in contradistinction to the prior art, the metal ions contained in the water cores of the impregnant solution are reduced directly on the support surface after impregnation of the micelle.
There are a number of recent reports in the literature concerning selective synthesis of hydrocarbons via certain Fischer-Tropsch type catalysts. For example, deviations from ASF distribution have been obtained by employing rethenium particles having narrow size distributions and encaged in Y-faujasites. See Nijs et al, J. Chemical Society Chemical Communications, page 180 (1979). Thus, a hydrocarbon product distribution was obtained with cutoffs at C.sub.3, C.sub.5 and C.sub.11 with 13 A, 20 A and 40 A Y-zeolite encaged ruthenium, respectively. It was reported that these deviations were due to the catalyst geometry imposing a size on the ruthenium particles. Similar deviations from ASF distribution have also been obtained with alumina supported, carbonyl derived iron particles. See Hughes et al, New Horizons in Catalysis, Kodansha Ltd., Tokyo, page 418, 1981. For these catalysts a cutoff carbon number of 4 was obtained with a very high selectivity at C.sub.2. With time, however, these particles sintered and lost their selectivity to light hydrocarbons. Finally, it has also been observed that the pore size distribution of a support may influence selectivity possibly by limiting the size of the active metal particle. See Vanhove et al, J. Chemical Society Chemical Communications, page 605, 1975. The results obtained with cobalt/alumina catalysts indicate that when the average pore diameter of the support was increased from 65 A to 300 A, the product distribution shifted towards nigher hydrocarbons: from C.sub.3 -C.sub.10 to C.sub.14 -C.sub.20.
Thus, evidence suggests that Fischer-Tropsch is a structure sensitive reaction and that selectivity is related to metal particle size.
The object of this invention therefore is to provide a catalytic composition of matter having a specified metal particle size range deposited thereon and its method of manufacture. Further, it is an object of the present invention to provide for a method of impregnating uniform Group VIII metal crystallites on a support material. In addition, it is also an object of the present invention to provide for an improved process for the conversion of synthesis gas comprising carbon monoxide and hydrogen to hydrocarbonaceous materials. Finally, it is also an object of the present invention to effect desired selectivity in the synthesis of hydrocarbons by controlling the geometry of the catalysts employed, in particular, the size of the metal particles deposited on the catalytic support. Further objects and embodiments of the present invention will become apparent to those skilled in the art and are intended to be included within the scope of the present invention.